Dog afraid of car

I just adopted a rescue dog. He's awesome and I love him, but the one thing is that he's terrified of getting in the car from a parking lot. I know that's weirdly specific. The day I brought him home from the shelter, he had my car scoped out from a surprisingly far distance, immediately did a "pancake" on the ground, and actually managed to get out of his collar and bolt. (Someone at the shelter grabbed him). I already knew the dog pretty well because I volunteer at the shelter and he's never been the smallest bit afraid of me–he follows me around all day, sleeps in my bed at night, etc. I figured I'd have some trouble getting him in the car when I adopted him, but nothing like that. Since then, I can get him in the car fairly easily from a garage to go places. I have to coax him in but he'll do it. I've been taking him to the park, and he's fine the whole time, not scared or timid or anything, but when we go to get BACK in the car it's the same thing. Pancake, trying to duck out of his collar, cowering…I'm terrified that he's going to get away again and this time I won't be able to get him back. Luring with food doesn't even help in that situation. He'll take food as long as can reach it without climbing in the car–and he's getting better and better at "reaching" food that's far away. Getting into a car from a parking lot seems to unlock a bad memory. Ordinarily I'd just wait and work on the car another time. But I'm at school and I need to get home at the end of the semester–which means driving him across the country in like six weeks. Any ideas?